Re: [xml-dev] Heed this warning about Postel's Prescription

[Michael Kay <mike@saxonica.com>]

Let’s just face it, there are arguments for Postel’s Law and there are arguments against.

XML has opted for the “be strict in what you accept” approach, which forces senders to be strict in what they transmit. The effect of that policy is that people who create garbage and call it XML find that no-one can use it. Hopefully this creates a feedback loop that forces them to change their ways, though there are often a lot of StackOverflow questions raised (by frustrated would-be recipients) en route to that goal.

If you go for the opposite “be liberal in what you accept” approach, senders will take less care over what they produce, and some recipients will be able to handle it and others won’t.

Which approach generates better interoperability in the long run? I don’t think anyone has a definitive answer to that question. But all my instincts tell me that I want to know the rules and I’m prepared to abide by them if it means that other people who do the same can communicate with me.

Michael Kay
Saxonica


> On 28 Jun 2015, at 20:59, Costello, Roger L. <costello@mitre.org> wrote:
> 
> How might Postel's Law be applied to web services that receive XML and sends out XML? 
> 
> Here are two ways:
> 
> 1. The web service is willing to receive UTF-8 XML documents containing a pseudo-BOM. The web service sends out UTF-8 XML documents without a pseudo-BOM. [1] 
> 
> 2. The web service is willing to receive XML character streams with Unicode decoding errors: it processes the character stream by replacing the offending bytes by the Unicode replacement character U+FFFD until it manages to resynchronize the UTF-{8,16} byte stream. The web service sends out XML documents without character decoding errors. [2] 
> 
> /Roger
> 
> [1] See Rick Jelliffe's post on the xml-dev list: http://lists.xml.org/archives/xml-dev/201506/msg00065.html 
> 
> [2] See Daniel Bunzli's post on the unicode list: http://www.unicode.org/mail-arch/unicode-ml/y2015-m06/0247.html